<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .father {
      width: 400px;
      height: 400px;
      background-color: purple;
      margin-top: 50px;
      /* border: 1px solid red; */
      /* border: 1px solid transparent; */
      /* padding: 1px; */
      overflow: hidden;
    }


    .son {
      width: 200px;
      height: 200px;
      background-color: green;
      margin-top: 100px;
    }
  </style>
</head>

<body>
  <div class="father">
    <div class="son"></div>
  </div>
</body>

</html>